ABSTRACT

Wild radish (Raphanus raphanistrum L.) is a prevalent annual weed throughout the cropping regions of southern Australia. A field experiment was conducted at Wagga Wagga, NSW to determine the effect of various densities and times of emergence of wild radish on canola yield and quality and on wild radish seed production. As few as 4 wild radish m^-2 emerging with canola reduced canola yield by 9%. A high wild radish density of 64 plants m^-2 emerging at 0, 2, 4, and 7 weeks after canola emergence reduced canola yield by 77, 54, 33, and 19%, respectively. Canola yield was not affected by wild radish when it emerged 10 weeks after the crop. Canola seed weight was reduced by up to 13% by wild radish that emerged 0, 2, and 4 weeks after that of canola. Wild radish seed yield and 1000-seed weight were greater when it emerged with canola than at later emergence times. Wild radish at 64 plants m^-2 that emerged 0, 2, 4, 7, and 10 weeks after canola emergence produced 24200, 11700, 7000, 1900, and 70 seeds m^-2, respectively. This large amount of wild radish seed could pose problems as a seed contaminant that would reduce canola quality and seed remaining in the field would perpetuate the weed problem in succeeding crops. Implications of these research findings on wild radish management in canola and rotational crops is discussed.

INTRODUCTION

Wild radish (Raphanus raphanistrum L.) is an annual weed that is widely distributed throughout the cropping areas of southern Australia (Cheam and Code 1995). Its ability to emerge and set seed at various times of the year and its high degree of seed dormancy make it difficult to control (Reeves et al. 1981). Wild radish is a vigorous competitor capable of causing large reductions in crop yield. Wild radish densities of 7 and 200 plants m^-2 have been reported to reduce wheat yield by 10 and 50%, respectively (Cheam and Code 1995). The effect of wild radish infestations on canola yield is not well documented. Additionally, because wild radish contains glucosinolates and erucic acid its presence as a contaminant in canola seed may markedly downgrade the quality of canola oil and meal.

The present study was conducted to determine the effect of various times of emergence and of various densities of wild radish on canola yield and quality. Wild radish seed production was determined as a measure its potential to contaminate canola seed and as an indication of its ability to infest succeeding crops.

MATERIALS AND METHODS

A field experiment was conducted on a Red Earth soil at Wagga Wagga, NSW in 1998 to examine wild radish interference in canola. Treatments consisted of various times of wild radish emergence relative to that of canola (0, 2, 4, 7 and 10 weeks after canola emergence), and at each emergence time, wild radish was established at densities of 4, 8, 16, 32, and 64 plants m^-2. A weed-free control also was included. The factorial set of treatments were arranged within a randomised complete block with three replicates. Individual plot size was 1.6 by 2 m.

Trifluralin at 1 kg ha^-1 was applied to the entire plot area before seeding for overall weed control and any weeds other than wild radish were removed by hand on a bi-monthly schedule. Oscar canola was sown at 4 kg ha^-1 in 20-cm rows in mid-May; resulting in densities ranging from 65 to 80 plants m^-2. Wild radish seed was broadcast on the soil surface by hand and lightly raked into the soil periodically throughout the growing season to achieve the desired emergence times relative to that of canola. Wild radish was counted and thinned to the desired densities at the cotyledon to one-leaf stage. Phenological growth stages of canola and wild radish emerging at the various times were recorded throughout the season. At canola maturity, the centre 1 m² portion of each plot was cut at ground level by hand. Canola and wild radish plants were put into separate jute bags and hung outdoors for two weeks to simulate gradual dry-down in a windrow. Canola was threshed in a stationary thresher and seed yield and 1000-seed weight were determined. Wild radish plants were further oven-dried at 35 C for five days and shoot dry weights recorded. Wild radish was subsequently threshed and seed yield, 1000-seed weight, and seed number were determined.

RESULTS AND DISCUSSION

Canola yield progressively decreased as wild radish density increased from 4 to 64 plants m^-2 when it emerged at 0, 2, 4, and 7 weeks after emergence of canola (Table 1). Wild radish that emerged 10 weeks after canola did not affect canola yield. Early emergence of wild radish increased its competitive ability with canola and greater reductions in canola yield resulted. As few as 4 wild radish m^-2 reduced canola yield by 9 and 4% when they emerged 0 and 2 weeks after canola. A high wild radish density of 64 plants m^-2 emerging at 0, 2, 4, and 7 weeks after canola emergence reduced canola yield by 77, 54, 33, and 19%, respectively. Canola seed weight also was reduced by up to 13% by wild radish that emerged 0, 2, and 4 weeks after that of canola.

Wild radish produced some viable seed even when it emerged 10 weeks after canola (mid-August) (Table 1). A study in Western Australia found that any wild radish plants that emerged in late July and onward were not able to reproduce (Panetta et al. 1988). The difference in results may be due to the earlier onset of drought conditions in Western Australia than in New South Wales.

Wild radish seed yield and 1000-seed weight were greater when it emerged with canola than at later emergence times (Table 1). Wild radish at 64 plants m^-2 that emerged 0, 2, 4, 7, and 10 weeks after canola emergence produced 24200, 11700, 7000, 1900, and 70 seeds m^-2, respectively. Reeves et al. (1981) found that 52 wild radish m^-2 in a wheat crop produced 17,275 seeds m^-2, indicating that the competitive ability of wild radish may be similar in wheat and canola. Wild radish seed contains high levels of glucosinolates and erucic acid. Thus, the large amounts of wild radish seed produced in this experiment potentially could reduce canola quality sufficiently to make the product virtually unsaleable. Wild radish seed remaining in the field would certainly pose a problem in succeeding crops.

Table 1. Effect of various wild radish densities and times of emergence on canola and wild radish seed yield parameters.

^a Weeks represents the number of weeks that wild radish emerged after canola and

density (4, 8, 16, 32, or 64) represents the number of wild radish plants m^-2.

CONCLUSIONS

Results indicate that wild radish can cause serious economic losses in canola. Herbicides to control wild radish can only be used in triazine-tolerant cultivars that inherently have a lower yield potential. Nevertheless, growers should consider this option if wild radish is prevalent on their farm. If herbicides are used to control wild radish they should provide sufficient residual control to kill late flushes of wild radish to reduce the risk of contaminating canola seed and perpetuating the weed problem in succeeding crops. Every effort should be given to reduce wild radish populations in crops preceding canola. A multi-year approach will be required to effectively manage wild radish in annual cropping systems.

REFERENCES

1. Cheam, A. H. and Code, G. R. 1995. The biology of Australian weeds. 24. Raphanus raphanistrum L. Plant Prot. Quart. 10:1-13.

2. Panetta, F. D., Gilbey, D. J. and D’Antuono, M. F. 1988. Survival and fecundity of wild radish (Raphanus raphanistrum L.) plants in relation to cropping, time of emergence and chemical control. Aust. J. Agric. Res. 39:385-397.

3. Reeves, T. G., Code, G. R. and Piggin, C. M. 1981. Seed production and longevity, seasonal emergence, and phenology of wild radish (Raphanus raphanistrum L.). Aust. J. Exp. Agric. Anim. Husb. 21:524-530.